Restoration circuit for returning the light output to normal upon the failure of one lamp



June 13, 1967 PETER DE H. EASTCOTT ETAL 3,325,683

RESTORATION CIRCUlT FOR RETURNING THE LIGHT OUTPUT TO NORMAL UPON THE FAILURE OF ONE LAMP Filed Jan, 13, 1965 INVENTORS PETER deH. EASTCOTT JAMES R. SPIBEY United States Patent 3,325,683 RESTORATION CIRCUIT FOR RETURNING THE LIGHT OUTPUT TO NORMAL UPON THE FAIL- URE OF ONE LAMP Peter de H. Eastcott and James R. Spibey, Peterborough, Ontario, Canada, assignors to Canadian General Electric Company, Limited, Toronto, Ontario, Canada, a limited company incorporated in Canada Filed Jan. 13, 1965, Ser. No. 425,139 Claims. (Cl. 315-258) This invention relates to a lighting circuit in which failure of a lamp causes other lamps to automatically increase their light output to compensate for the lamp lost.

In some lighting applications it is desirable to maintain the light output near a constant level for a definite period of time even though a lamp burns out. An example of a lighting system of this kind is the one used in the web handling system described in the applicants copending application, 'Ser. No. 425,301, filed Jan. 13, 1965, now abandoned. If a lamp in this application burns out, it will not be replaced until the system is shut down for regular maintenance. Consequently, the system must include some means to keep its light output near the former level until the burned out lamp can be replaced during the next shut down period.

This invention provides a simple means for holding the light output from a bank of incandescent lamps nearly constant even after a lamp in the bank has burned out. This is accomplished by placing across a constant voltage source a parallel bank of incandescent lamps in series with a resistor having a resistance value suitably related to the resistance of the bank of lamps. The resistance value of the resistor is selected such that the lamps normally operate well below their rated voltage; upon failure of a lamp, the voltage across the remaining lamps rises sufficiently to increase their light output to approximately the former level.

A specific embodiment of the invention will now be described with reference to the single drawing figure attached.

In the circuit shown in the drawing the numeral 2 designates a conventional voltage stabilizing transformer having its primary 3 connected to a regular 115 volt alternating current supply and its secondary 4 connected to a lamp load maintained at a constant voltage. The lamp load consists of the two banks of lamps 5 and 6 connected in series with resistors 9 and respectively, bank 5 having lamps 11 and 12 in parallel and bank 6 lamps 7 and 8 in parallel. For the best results the lamps in each bank should be of the same type and rating, that is, lamps 11 and 12 should be as near the same as manufacturing tolerances will normally permit. The same applies for lamps 7 and 8.

An examination of the circuit will show that the current flowing through resistor 9 is equal to the sum of the currents flowing through lamps 11 and 12. This means that the voltage output from the secondary winding 4 will be shared between the bank of lamps 5 and the resistor 9 in proportion to the resistance of each. Should a lamp in bank 5 burn out, the current flowing to resistor 9 will decrease because the total resistance of the bank has increased, thereby reducing the voltage drop across the resistor. Since the voltage across the parallel-series combination of 11, 12 and 9 is held constant, a decrease in voltage across resistor 9 will be accompanied by an increase in voltage across the remaining lamp in bank 5. If, when both lamps are burning, they operate well below their rated voltage, a rise in voltage across one lamp due to failure of the other will automatically increase the light output from the good lamp. Since the bank of lamps 6 and resistor 10 constitute the same kind of circuit, the

resistance of resistor 10 will be related to the resistance of lamps 7 and 8 in the way described in connection with resistor 9 and lamps 11 and 12. That is, the characteristics and operation of the two circuits are the same. More than two banks of lamps and associated resistors may, of course, be used in the practice of the invention.

When two lamps are placed in parallel in a bank, the value of the resistance chosen to be placed in series with the bank will reduce the voltage across the lamps below rated voltage to the point where each lamp gives a light output approximately equal to one-half the light obtained from the lamp at rated voltage. Failure of a lamp in the bank causes the voltage across the remaining lamp to rise to near rated, thereby increasing its light output to approximately that normally obtainable at rated voltage. Stated in other words, the failure of a lamp in the bank causes the good lamp to approximately double its light output, and thereby maintain the total light from the bank near constant.

It is to be understood that if both lamps in a bank such as 5 or 6 fail, the total light output will diminish, and the defective lamps may have to be replaced immediately. The chances of this happening are slight compare-d with the chances of one of the lamps in a bank failing. Lamps arranged in a circuit according to the invention avoids immediate replacement of lamps due to the failures most likely to occur; it does not eliminate altogether the need for immediate replacement of lamps. The web handling system described in the aforementioned application for patent will be shut down periodically for maintenance and repair, usually once a week. During this period, the optical equipment will be inspected and any burned out lamps replaced.

Although the drawing and the description thereof relate to a circuit employing but two lamps per bank, the invention is not so limited. A bank may contain more than two lamps connected in parallel with the parallel combination connected in series across a voltage source. If more than two lamps are employed per bank, the series resistance will be chosen accordingly; its value will be such that failure of a lamp in the bank will cause the voltage across the remaining lamps to rise to the point where they keep the total light output of the bank at approximately its former level.

Details of an actual application of the invention will now be given. In this example, the primary of a conventional voltage stabilizing transformer was energized .from a standard -125 volt A.C. outlet, and a constant secondary voltage of 6.3 volts was obtained. Six sealed beam automobile headlamps were used as the light source; each lamp was rated 30 watts at 6 to 8 volts with 300 hours average life. The lamps were arranged in three banks of two lamps each in parallel and a resistor in series with each bank, and the parallel-series combination was con nected across the secondary of the transformer. Heavy filament, low voltage lamps were selected because they give steadier light output on AC. than do lamps with thin filaments, and they also stand up better under vibrations. Obviously, a steady light output can be obtained from any incandescent lamp by converting the AC. to DC. before it is applied to the lamp. Curves showing light output of a lamp plotted against filament voltage are available from the lamp manufacturer.

The resistor used with each bank of lamps had an overall resistance of about one tenth ohm and a slider for selecting a portion of this resistance for connection in series with the bank across the secondary of the transformer. To select the proper value of the resistance, the light output from the bank was measured by means of a light meter, first with both lamps burning and then with one lamp burning. After each pair of readings, the slider on the resistor was reset to increase or decrease the resistance depending on the meter readings. This procedure was repeated until the meter showed the same light output from either the two lamps burning simultaneously or one lamp burning alone.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A lighting system adapted to maintain a near constant light output with failure of a lamp in the system comprising a constant voltage source; a bank of at least two lamps connected in parallel and the parallel combination connected in series with a resistor across said voltage source; each lamp having a rated voltage equal to or a little less than the voltage of the source, and said resistor having a resistance value such that the voltage across the bank of lamps is reduced to the point where each lamp operates below its rated light output and failure of a lamp causes the voltage across the remaining lamps to increase to the extent that they maintain near constant the level of the light from the bank.

2. A lighting system adapted to maintain a near constant light output with failure of a lamp in the system comprising a constant voltage source; a bank of two identical lamps connected in parallel and the parallel combination connected in series with a resistor across said voltage source; each lamp having a rated voltage equal to or a little less than the voltage of the source, and said resistor having a resistance value such that the voltage across the bank of lamps is reduced to the point where each lamp operates at about half its rated light output, and failure of one lamp causes the voltage across'the other lamp to increase to near rated voltage to substantially double the light output from said other lamp.

3. The lighting system defined in claim 1 wherein said source is alternating current and said resistor is replaced with an inductive reactance.

4. The lighting system defined in claim 1 wherein said source is alternating current and said resistor is replaced with an impedance.

5. The lighting system defined in claim 1 wherein a plurality of said banks of lamps are used, each bank having a separate resistor in series therewith and the combination being connected across the constant voltage source.

References Cited UNITED STATES PATENTS 2,161,443 6/1939 Warshawsky 315192 X 3,155,952 11/1964 Boehm et a1. 315-192 X FOREIGN PATENTS 534,641 1/ 1922 France.

JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner. 

1. A LIGHTING SYSTEM ADAPTED TO MAINTAIN A NEAR CONSTANT LIGHT OUTPUT WITH FAILURE OF A LAMP IN THE SYSTEM COMPRISING A CONSTANT VOLTAGE SOURCE; A BANK OF AT LEAST TWO LAMPS CONNECTED IN PARALLEL AND THE PARALLEL COMBINATION CONNECTED IN SERIES WITH A RESISTOR ACROSS SAID VOLTAGE SOURCE; EACH LAMP HAVING A RATED VOLTAGE EQUAL TO OR A LITTLE LESS THAN THE VOLTAGE OF THE SOURCE, AND SAID RESISTOR HAVING A RESISTANCE VALVE SUCH THAT THE VOLTAGE ACROSS THE BANK OF LAMPS IS REDUCED TO THE POINT WHERE EACH LAMP OPERATES BELOW ITS RATED LIGHT OUTPUT AND FAILURE OF A 